Dye-sensitized solar cell based on electrospun ultra-fine titanium dioxide fibers and fabrication method thereof

ABSTRACT

A dye-sensitized solar cell comprising a semiconductor electrode comprising electrospun ultra-fine titanium dioxide fibers and fabrication method thereof are disclosed. The dye-sensitized solar cell comprises a semiconductor electrode comprising an electrospun ultra-fine fibrous titanium dioxide layer, a counter electrode and electrolyte interposed therebetween. A non-liquid electrolyte such as polymer gel electrolyte or the like having low fluidity, as well as the liquid electrolyte, can be easily infiltrated thereinto. In addition, electrons can be effectively transferred since titanium dioxide crystals are one-dimensionally arranged.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a dye-sensitized solar cell to convertsolar energy into electric energy by a photo-electrochemical process andto a fabrication method thereof, and particularly, to a dye-sensitizedsolar comprising a semiconductor electrode consisting of electrospunultra-fine titanium dioxide fibers and to a fabrication method thereof.

2. Description of the Background Art

Since Grätzel's research group in Swiss has reported a dye-sensitizedsolar cell (B. O'Regan, M. Grätzel, Nature 353, 737 (1991)), researchesthereinto have been actively conducted. The dye-sensitized solar cell byGrätzel et al. is a photo-electrochemical solar cell using an oxidesemiconductor electrode comprising photosensitive dye molecules that canabsorb light within visible region, thereby to generate an electron-holepair and nano crystalline titanium dioxide that can transfer thegenerated electron. In this cell, electrons excited in dye moleculesupon receiving light within visible region are transferred to titaniumdioxide which is an n-type semiconductor, and dye molecules arereproduced through an electrochemical oxidation-reduction of I⁻/I₃⁻contained in a liquid electrolyte, by which current is generated.

The dye-sensitized solar cell is much expected as a solar lightconversion element due to its higher energy conversion efficiency, whileits fabrication cost is relatively low compared to a conventionalsilicon solar cell.

However, because dye-sensitized solar cells provided until now include aliquid electrolyte, stability problems have been raised, and especially,because it is difficult for such solar cells to be sealed, the liquidelectrolyte can be leaked or an electrochemical stability cannot beensured in using the same for a long time. Recently, in order to resolvesuch problems, researches have been actively conducted for using aninorganic solid electrolyte (Langmuir 19, 3572 (2003)), a polymer solidelectrolyte (Electrochemica Acta 47, 2801 (2002)), a gel electrolyte (J.Phys, Chem. B 107, 4374 (2003)), an ionic liquid (J. Am. Chem. Soc. 125,1166 (2003)), an organic hole carrier (Science 295, 2425 (2002)) or thelike, instead of using a liquid electrolyte. However, while a liquidelectrolyte can easily infiltrate throughout an entire electrode platehaving a thickness of 10 μm or more which is fabricated by sinteringnanocrystalline titanium dioxide particles, it is difficult for anon-liquid electrolyte to infiltrate into such electrode plate, andtherefore, energy conversion efficiency is lowered for cells using anon-liquid electrolyte compared to those using a liquid electrolyte(Chem. Lett. 30, 26 (2001); 31, 948 (2002)). Although a cellconstruction using a porous titanium dioxide thin film based on asol-gel method or particles in rod shape has been proposed as a solutionfor such problems, it has been known that its performance is muchinferior compared to the conventional nano-particle type. Therefore,there are problems yet to be solved.

SUMMARY OF THE INVENTION

Therefore, one object of the present invention is to provide adye-sensitized solar cell comprising a semiconductor electrodeconsisting of electrospun ultra-fine fibrous crystalline titaniumdioxide, which can solve electron mobility problems throughout particlesoccurring in the conventional dye-sensitized solar cell using anelectrode made of sintered nanocrystalline titanium dioxide, and whichcan also improve adhesiveness with solid electrolyte and elementcharacteristics.

Another object of the present invention is to provide a method forfabricating the dye-sensitized solar cell comprising electrospunultra-fine fibrous titanium dioxide.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention andtogether with the description serve to explain the principles of theinvention.

In the drawings:

FIG. 1 illustrates the construction of a dye-sensitized solar cell inaccordance with the present invention;

FIG. 2 is a schematic view of a general type of an electrospinningapparatus;

FIGS. 3A to 3C are scanning electron microscopic photographs of anelectrospun ultra-fine fibrous titanium dioxide layer fabricated inaccordance with the present invention (FIG. 3A: before pre-treatment;FIG. 3B: after pre-treatment but before thermal treatment; and FIG. 3C:after thermal treatment);

FIG. 4 is a transmission electron microscopic photograph of anelectrospun ultra-fine fibrous titanium dioxide layer fabricated inaccordance with the present invention;

FIGS. 5A and 5B are photographs of a transparent conductive substrateafter thermal treatment, on which an electrospun ultra-fine fibroustitanium dioxide layer was formed in accordance with the presentinvention (FIG. 5A: without performing pre-treatment; and FIG. 5B: withperforming pre-treatment);

FIG. 6 is a graph showing X-ray diffraction peaks for an electrospunultra-fine fibrous titanium dioxide prepared in accordance with thepresent invention after thermal treatment; and

FIG. 7 is a graph showing current-voltage characteristics of adye-sensitized solar cell in accordance with the present invention.

-   -   10: Electrode    -   11, 21: Glass    -   12, 22: Transparent FTO conductive layer    -   13: Electrospun titanium dioxide layer    -   20: Counter electrode    -   23: Pt layer    -   30: Electrolyte    -   40: Spacer

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The inventors of the present invention has made great efforts to solvethe problems in the conventional dye-sensitized solar cell and finallydeveloped an semiconductor electrode consisting of an electrospunultra-fine fibrous titanium dioxide, so as to provide a newdye-sensitized solar cell having a titanium dioxide layer in onedimensional structure that can facilitate infiltration of a non-liquidelectrolyte and can effectively transfer electrons.

To achieve the above-mentioned objects and other advantages inaccordance with the present invention, as embodied and broadly describedherein, there is provided a dye-sensitized solar cell comprising asemiconductor electrode containing an electrospun ultra-fine fibroustitanium dioxide layer; a counter electrode; and electrolyte infiltratedbetween the semiconductor electrode and the counter electrode.

The semiconductor electrode consists of a glass substrate, ITO or FTOtransparent conductive layer, and an electrospun ultra-fine fibroustitanium dioxide layer onto which dye molecules are adsorbed. Theelectrospun ultra-fine fibrous titanium dioxide layer has a thickness of5-20 μm.

The counter electrode can include a glass substrate, ITO or FTOtransparent conductive layer and a platinum layer.

The electrolyte is a liquid electrolyte containing iodine, andpreferably, an electrolyte of 0.1M lithium iodide (LiI), 0.05M iodine(I₂), 0.6M 1,2-dimethyl-3-propyl-imidazolium iodide and 0.5 M tert-butylpyridine in acetonitrile, or a polymer gel electrolyte containing atleast one polymers selected from the group consisting ofpoly(vinylidenefluoride)-co-poly(hexafluoropropylene),poly(acrylonitrile), poly(ethyleneoxide) and poly(alkylacrylate).Preferably, the polymer gel electrolyte contains one or more polymers asmentioned above in an amount of 5-20% by weight of a mixture ofpropylene carbonate and ethylene carbonate.

To achieve another object of the invention, there is also provided amethod for fabricating a dye-sensitized solar cell comprising the stepsof:

-   -   adding titanium isopropoxide into a polymer solution, followed        by adding acetic acid as a catalyst therein, and then stirring        the resulting mixture at room temperature, to obtain a solution        for electrospinning;    -   electrospinning the solution to form a film made of ultra-fine        titanium dioxide fibers onto an ITO or FTO-coated transparent        conductive glass substrate;    -   pre-treating the substrate having the film comprising the        ultra-fine titanium dioxide fibers formed thereon with acetone        or dimethyl formamide;    -   thermally treating the pre-treated substrate to form a layer of        the ultra-fine fibrous titanium dioxide onto the substrate;    -   impregnating the thermally treated substrate in a solution of        dye molecules in ethyl alcohol, to obtain a semiconductor        electrode in which the dye molecules are adsorbed into the        electrospun ultra-fine titanium dioxide fibers;    -   coating a platinum layer onto an ITO or FTO-coated transparent        conductive glass substrate, to obtain a counter electrode;    -   performing a heating/pressing process on the semiconductor        electrode and the counter electrode in which a spacer having a        thickness of 20 μm is located therebetween, so as to attach the        above two electrodes to each other; and    -   injecting an electrolyte into the empty space between the        semiconductor electrode and the counter electrode.

The ultra-fine titanium dioxide fibers are electrospun so as to have50-1000 nm in thickness.

The pre-treatment can be performed such a manner that i) the substrateis pre-processed with vapor of acetone or dimethylformamide for 1-3hours in a closed container; ii) the substrate is immersed in acetone ordimethylformamide solvent for 1 hour, or iii) methods i) and ii) arecombined. Preferably, the thermal treatment is performed at 450° C.-500°C. for 30 minutes.

Below, the present invention will be described in more detail byreferring to the attached drawings.

FIG. 1 illustrates the construction of a dye-sensitized solar cellcomprising a semiconductor electrode consisting of electrospunultra-fine titanium dioxide fibers in accordance with the presentinvention.

With reference to FIG. 1, the dye-sensitized solar cell comprises asemiconductor electrode 10, a counter electrode 20 and an electrolyte 30interposed therebetween.

The semiconductor electrode 10 comprises a glass substrate 11, an indiumtin oxide (ITO) or fluorine-doped tin oxide (FTO) transparent conductivelayer 12 and an electrospun ultra-fine fibrous titanium dioxide layer13. The ultra-fine fibrous titanium dioxide layer 13 includes ultra-finetitanium dioxide fibers having a diameter of about 50-1000 nm which areformed with a bunch of fine fibrins having a thickness of about 10-30 nmone-dimensionally arranged by electrospinning. In order to effectivelygenerate photocurrent, the titanium dioxide layer 13 preferably has athickness of about 5-20 μm. Ruthenium-based dye molecules are adsorbedonto the ultra-fine titanium dioxide fibers.

The counter electrode 20 comprises a glass substrate 21, an ITO or FTOtransparent conductive layer 22 and a platinum layer 23. The platinumlayer 23 of the counter electrode 20 is disposed to face the fibroustitanium dioxide layer 13 of the semiconductor electrode 10.

As the electrolyte 30 filled in the space between the semiconductorelectrode 10 and the counter electrode 20, an electrolyte solution of I₃⁻/I⁻ in which 0.1M lithium iodide (LiI), 0.05M iodine (I₂), 0.6M1,2-dimethyl-3-propyl-imidazolium iodide and 0.5M tert-butyl pyridineare dissolved in acetonitrile. Alternatively, instead of the liquidelectrolyte, a polymer gel electrolyte in which one or more polymersselected from the group consisting ofpoly(vinylidenefluoride)-co-poly(hexafluoropropylene),poly(acrylonitrile), poly-(ethyleneoxide) and poly(alkylacrylate) arecontained in an amount of 5-20% by weight of a mixture of propylenecarbonate and ethylene carbonate can be used.

A method for fabricating a dye-sensitized solar cell comprising asemiconductor electrode consisting of electrospun ultra-fine titaniumdioxide fibers in accordance with the present invention will bedescribed below.

In order to fabricate the semiconductor 10, an anode, the electrospunultra-fine titanium dioxide fibers are fabricated.

To begin with, a solution for an electrospinning is prepared by asol-gel reaction of titanium isopropoxide. In detail, polyvinylacetatehaving an excellent affinity to titanium dioxide is dissolved indimethylformamide, acetone, tetrahydrofuran, toluene or a mixturethereof, to prepare a 5-20 wt % polymer solution to be able to give aviscosity suitable for electrospinning. Polyvinylacetate having a weightaverage molecular weight of 100,000-1,000,000 g/mol is used. Instead ofpolyvinylacetate, polyvinylpyrrolidone, polyvinylalcohol,polyethyleneoxide and the like can be used to prepare a polymersolution. Next, titanium isopropoxide is added into the polyvinylacetatepolymer solution in an amount of 5-25 wt % of the polyvinylacetatepolymer solution, to which acetic acid is added as a catalyst in anamount of 20-60 wt % of the titanium isopropoxide. The resulting mixturewas then reacted for 1-5 hours at room temperature so as to obtain asolution for electrospinning. This solution is required to maintain asuitable viscosity required for electrospinning. After being spun intofibers, the polymer is completely decomposed by a thermal treatment at450° C. or higher, and residual titanium dioxide is converted into ananatase type crystal structure.

And then, electrospun ultra-fine titanium dioxide fibers are obtainedwith an electrospinning apparatus. As shown in FIG. 2, a generally usedelectrospinning apparatus includes a spinning nozzle connected to aquantizing pump that can introduce a solution to be spun quantitatively,a high voltage generator, an electrode for forming a layer of fibers tobe spun. An earthed transparent conductive glass substrate, specificallya transparent conductive glass substrate onto which ITO or FTO arecoated and which have conductivity of 5-30 Ω is used as an anode, andthe spinning nozzle equipped with a pump which can control dischargingamount per time is used as a cathode. 10-30 KV of voltage is applied anda solution discharge rate is adjusted at 10-50 I/min, so as to obtainultra-fine titanium dioxide fibers having a thickness of 50-1,000 nm.Electrospinning is continued until a film consisting of ultra-finetitanium dioxide fibers at a thickness of 5-20 μm is formed onto thetransparent conductive substrate. By adjusting discharging amount andvoltage, the thickness and form of the fiber can be controlled. Inaddition, in order to uniformly maintain the thickness of the filmentirely, it is preferred to use a robot system which can repeatedlymoves the position of the electrospinning nozzle.

Thereafter, before thermal treatment, a pre-treatment is performed suchthat the transparent conductive substrate, onto which the film made ofthe electrospun ultra-fine titanium dioxide fibers is formed, is treatedwith vapor of acetone or dimethylformamide, which has been used assolvent for the polymer solution, for 1-3 hours in a closed container orimmersed the substrate in acetone or dimethyl formamide for 1 hour. Theelectrospun ultra-fine titanium dioxide fibers are in the form of a filmmixed with a polymer on the substrate (referred to as ‘polymer-titaniumdioxide composite film’, hereinafter). Thus, in order to obtain thesemiconductor electrode in accordance with the present invention, it isnecessary for the substrate on which electrospun titanium dioxide filmis formed to be subjected to a thermal treatment at a temperature of450° C. or higher, thereby to remove the polymer binder completely andto convert residual ultra-fine titanium dioxide fibers into anatase typecrystal structure. However, if a polymer-titanium dioxide composite filmformed by a conventional electrospinning is thermally treated at such ahigh temperature in the air, the transparent conductive substrate andthe titanium dioxide film are separated (refer to FIG. 5A), andtherefore, it can not be used as an electrode for a preferabledye-sensitized solar cell.

Therefore, in the present invention, the polymer-titanium dioxidecomposite film formed by electrospinning is subjected to a pre-treatmentso as to form a firm titanium dioxide film (refer to FIG. 5B). Thepre-treatment is to dissolve the polymer portion of the polymer-titaniumdioxide composite fibers by treat the electrospun ultra-fine titaniumdioxide fibers in which polymer and titanium dioxide are mixed (referredto as ‘polymer-titanium dioxide composite fiber’, hereinafter) with asolvent. During this process, adhesiveness among fibers can beincreased, while the fibrous form of the titanium dioxide is maintainedas it is, so that its adhesiveness to the lower transparent conductivesubstrate can be increased. As stated above, the pre-treatment includesthe method of treating the electrospun polymer-titanium dioxidecomposite film with vapor of acetone or dimethylformamide used as asolvent for the polymer solution for 1-3 hours, the method of immersingthe electrospun polymer-titanium dioxide composite film in acetone ordimethyl formamide, and the method of employing both methods. In theaspect of efficiency, it is preferred to use the third method in amanner that the electrospun polymer-titanium dioxide composite film istreated with vapor and then immersed in the solvent.

The pre-processed transparent conductive substrate is thermally treatedat 450-500° C. for 30 minutes in the air to thermally decompose residualpolymer so as to completely remove the same, by which crystallinestructure of the titanium dioxide is converted into anatase type.

And then, the substrate, on which the film made of the electrospuntitanium fibers is formed, is impregnated in a solution in which aRuthenium-based dye molecules, for example, dye molecules represented bya structural formula of RuL₂(NCS)₂, whereinL=2,2′-bipyridyl-4,4′-dicarboxyl acid, is dissolved in ethyl alcohol ina concentration of 3×10⁻⁴M for 12 hours or more, to adsorb the dyemolecules therein. The substrate was then washed with ethyl alcohol andthen dried, to obtain the semiconductor electrode 10 comprisingdye-adsorbed electrospun ultra-fine titanium dioxide fibers.

Subsequently, in order to fabricate the counter electrode 20, which is acathode, the platinum layer 23 is coated onto an ITO or FTO-coatedtransparent conductive glass substrate.

And then, the counter electrode 20, the cathode, and the semiconductorelectrode 10, the anode, are assembled such that each conductive surfaceof the cathode and anode comes inward so as to make the platinum layer23 and the fibrous titanium dioxide layer 13 faced with each other. Atthis time, the two electrodes are attached with a spacer 40 having athickness of about 20 μm, which is made of a thermoplastic Surlyn™(available from Du Pont Co.) and inserted the above two electrodes.

Thereafter, a liquid electrolyte or polymer gel electrolyte is filled inspaces between the two electrodes. As mentioned above, as the liquidelectrolyte, the electrolyte solution of I₃ ⁻/I⁻-obtained by dissolving0.1M of lithium iodine (LiI), 0.05M of iodine (I₂), 0.6M of1,2-dimethyl-3-propyl-imidazolium iodide and 0.5M of tert-butyl pyridinein acetonitrile. As the polymer gel electrolyte, a mixture obtained bydissolving one or more polymers selected from the group consisting ofpoly(vinylidenefluoride)-co-poly(hexafluoropropylene),poly(acrylonitrile), poly(ethyleneoxide) and poly(alkylacrylate) in amixture of propylene carbonate and ethylene carbonate in amount of 5-20wt % of the solvent mixture can be used.

The present invention will be explained in more detail in the followingexamples. It is to be understood that these examples are merelyillustrative and it is not intended to limit the scope of the presentinvention to these examples, and they can be modified by the ordinaryperson skilled in the art within the scope.

EXAMPLE Example 1 Fabrication of an Electrospun Titanium Dioxide FiberLayer

6 g of titanium isopropoxide was slowly added in a polymer solution inwhich 30 g of polyvinylacetate (Mw 500,000, a product of Aldrich Co.)was dissolved in a 270 m of acetone and 30 ml of dimethylformamide mixedsolvent. As the reaction was initiated by moisture contained in thesolvent, the reaction mixture was changed into a suspension. 2.4 g ofacetic acid was then slowly added dropwise as a reaction catalyst to thereaction mixture. As the reaction was proceeding, the suspension waschanged into a clear solution. The resulting spinning solution should bespun to ultra-fine titanium dioxide fibers within 24 hours once it wasprepared, because if it is left for a long time after the acetic acidwas added, the solution is changed into a dark brown color due tohydrolysis of the polymer.

Electrospinning was performed with the electrospinning apparatus shownin FIG. 2, wherein a FTO-coated transparent conductive substrate (havingthe size of 10 cm×10 cm) was used as a cathode and a metallic needle(No. 24) having a pump, which can control discharge rate, attachedthereto was used as an anode, to which 15 kV of voltage was applied.While discharge rate of the spinning solution was maintained at 30μl/min, electrospinning was performed until the total discharge amountof the spinning solution reaches 5,000 μl, to form a layer of ultra-finetitanium dioxide fibers onto the FTO-coated transparent conductivesubstrate.

Example 2 Pre-Treatment and Thermal Treatment of the Substrate on Whicha Layer of Titanium Dioxide Fibers was Formed Fabricated in Example 1

The layer of titanium dioxide fibers fabricated in Example 1 includespolymer and titanium dioxide mixed therein. Thus, in order to use thepolymer-titanium dioxide composite film-formed substrate as asemiconductor electrode of a dye-sensitized solar cell, the substrateshould be thermally treated at a high temperature to removepolyvinylacetate, a polymer binder, and the spun ultra-fine titaniumdioxide fibers should be converted into a crystal form. However, if thesubstrate fabricated in Example 1 is thermally treated at a hightemperature without pre-treatment, the titanium dioxide film would notbe attached on the FTO-coated substrate but be separated therefrom, andthus, it cannot be used as a semiconductor electrode for adye-sensitized solar cell.

Thus, before the thermal treatment, the substrate, on which thepolymer-titanium dioxide composite film fabricated in Example 1 has beenformed, was treated only with acetone vapor for 1 hour, without beingdirectly contacted with acetone in a closed container, and then,thermally treated in an electric furnace of 450° C. in the air for 30minutes, thereby to stably form titanium dioxide film on the FTO-coatedtransparent conductive substrate.

FIGS. 3A to 3C are scanning electron microscopic photographs of theelectrospun titanium dioxide fiber layer in stages fabricated inaccordance with the present invention. Specifically, FIG. 3A is ascanning electron microscopic photograph of the titanium dioxide fiberlayer after electrospinning but before pre-treatment, FIG. 3B is ascanning electron microscopic photograph of the titanium dioxide fiberlayer after pre-treatment, and FIG. 3C is a scanning electronmicroscopic photograph of the titanium dioxide fiber layer afterpre-treatment and thermal treatment.

As shown in FIG. 3A, in the titanium dioxide fiber layer that wasthermally treated without pre-treatment, individual fibers were fixedseparately without being adhered to each other after thermal treatment.When the titanium dioxide fiber layer was subjected to thepre-treatment, fibers were well connected with each other to form adense film as shown in FIG. 3B because the matrix polymer from fibersspun are partially dissolved.

The ultra-fine fibrous titanium dioxide layer obtained after thepre-treatment and subsequent thermal treatment shows that the layer ofindividual fibers were well adhered to each other, so as to enableelectron transfer to be more effective in a solar cell device.

FIG. 4 is a transmission electron microscopic photograph of theelectrospun titanium dioxide fiber layer fabricated in accordance withthe present invention, and especially shows fine structure of theultra-fine titanium dioxide fibers remaining after removing polymer bythe pre-treatment and subsequent thermal treatment at 450° C. As shownin FIG. 4, the fibrous titanium dioxide crystal arrangedone-dimensionally increases movement photocurrent in the solar cell,which are intended by the present invention, thereby to remarkablyimprove performance of the element.

FIGS. 5A and 5B are photographs of transparent conductive substrate withthe electrospun titanium dioxide fiber layer formed thereon after thesubstrate was thermally treated in accordance with the presentinvention. FIG. 5A is for the substrate without performing thepre-treatment thereon, and FIG. 5B is for the pre-treated substrate.

As shown in FIG. 5A, when the substrate was thermally treated at 450° C.without pre-treatment, the titanium dioxide film was separated from thesubstrate. On the contrary, when the substrates was pre-treated withvapor of acetone, followed by thermally treated, the titanium dioxidefilm was firmly formed on the substrate as shown in FIG. 5B.

FIG. 6 is a graph showing X-ray diffraction peaks after the electrospunultra-fine titanium dioxide fibers were thermally treated in accordancewith the present invention. It can be seen from FIG. 6 that thecrystalline structure of the titanium dioxide was converted into theanatase type after the thermal treatment.

Example 3 Fabrication of a Dye-Sensitized Solar Cell Using the TitaniumDioxide Fiber Layer Fabricated in Example 2

Dye molecules were adsorbed onto the ultra-fine titanium dioxide fibersin the substrate fabricated in Example 2. Specifically, the transparentconductive glass substrate fabricated in Example 2 was impregnated in3×10⁻⁴M solution of RuL₂(NCS)₂ (L=2,2′-bipyridyl-4,4′-dicarboxyl acid)(Ruthenium 535, available from Solaronix,), a ruthenium-based dye, inethanol for 12 hours, so as to adsorb the dye molecules therein. Theresulting substrate was washed with ethanol several times and thendried, thereby to give a semiconductor electrode. Separately, a platinumlayer was coated onto a FTO-coated transparent conductive glasssubstrate to obtain a counter electrode.

Next, a spacer having a thickness of about 20 μm was located between thesemiconductor electrode and the counter electrode fabricated asdescribed above, and a certain pressure was applied thereto at 120° C.so as to attach the above two electrodes. Iodine-based liquidelectrolyte was then filled in the space between the above twoelectrodes, and the resultant was sealed to obtain the dye-sensitizedsolar cell in accordance with the present invention. Liquid electrolyteused was prepared by dissolving 0.25 g of iodine, 0.26 g of lithiumiodide, 3.70 g of 1,2-dimethyl-3-propyl-imidazolium iodide and 1.34 g oftert-butyl pyridine in 20 ml of acetonitrile.

Meanwhile, the dye-sensitized solar cell was also fabricated in the samemanner as described in Example 1 to Example 3, except for using apolymer gel electrolyte instead of the liquid electrolyte. The polymerget electrolyte used in this case was prepared by dissolving 0.125 g ofpoly(vinylidene fluoride)-co-poly(hexafluoropropylene) (Kynar 2801),0.13 g of 1-hexyl-2,3-dimethyl imidazolum iodide (Im), and 0.008 g ofiodine in a mixture of 0.75 g of propylene carbonate and 0.5 g ofethylene carbonate at 80° C.

FIG. 7 is a graph showing current-voltage characteristics of thedye-sensitized solar cell fabricated in the above Examples in accordancewith the present invention. As an electrolyte for the dye-sensitizedsolar cell, the liquid electrolyte and the gel electrolyte prepared asdescribed in Example 3 were respectively used. Table 1 below showsphotoelectrochemical characteristics which were calculated withcurrent-voltage curves relative to the types of the electrolytes,namely, photocurrent density (J_(SC)), voltage (V_(OC)), fill factor(ff) and energy conversion efficiency (η). TABLE 1 Electrolytes J_(sc)(mA/cm²) V_(oc) (V) ff η (%) Liquid electrolyte 8.35 0.76 0.62 3.90 Getelectrolyte 7.72 0.80 0.58 3.61

As can be seen from the above Table 1, the dye-sensitized solar cellcomprising a semiconductor electrode consisting of the electrospunultra-fine titanium dioxide fibers according to the present inventionexhibits energy conversion efficiency of above 90% even in using gelelectrolyte relative to that of the cell using liquid electrolyte.

As so far described, the dye-sensitized solar cell in accordance withthe present invention comprises a semiconductor electrode comprisingelectrospun ultra-fine titanium dioxide fibers, and therefore, thenon-liquid electrolyte such as a polymer gel electrolyte or the likehaving low fluidity, as well as a liquid electrolyte, can even be easilyinfiltrated thereinto. In addition, electrons can be effectivelytransferred because titanium dioxide crystals are arrangedone-dimensionally.

As the present invention may be embodied in several forms withoutdeparting from the spirit or essential characteristics thereof, itshould also be understood that the above-described embodiments are notlimited by any of the details of the foregoing description, unlessotherwise specified, but rather should be construed broadly within itsspirit and scope as defined in the appended claims, and therefore allchanges and modifications that fall within the metes and bounds of theclaims, or equivalence of such metes and bounds are therefore intendedto be embraced by the appended claims.

1. A dye-sensitized solar cell comprising: a semiconductor electrodecomprising an electrospun ultra-fine fibrous titanium dioxide layer; acounter electrode; and an electrolyte interposed between thesemiconductor electrode and the counter electrode.
 2. The cell accordingto claim 1, wherein the semiconductor electrode comprises a glasssubstrate, a ITO or FTO transparent conductive layer and the electrospunultra-fine fibrous titanium dioxide layer onto which dye molecules areadsorbed.
 3. The cell according to claim 1, wherein the electrospunultra-fine fibrous titanium dioxide layer has a thickness of 5-20 μm. 4.The cell according to claim 1, wherein the counter comprises a glasssubstrate, an ITO or FTO transparent conductive layer and a platinumlayer.
 5. The cell according to claim 1, wherein the electrolyte is aliquid electrolyte containing iodine.
 6. The cell according to claim 5,wherein the liquid electrolyte is an electrolyte in which 0.1M lithiumiodide, 0.05M iodine, 0.6M 1,2-dimethyl-3-propyl-imidazolium iodide and0.5M tert-butyl pyridine are dissolved in acetonitrile.
 7. The cellaccording to claim 1, wherein the electrolyte is a polymer gelelectrolyte containing one or more polymers selected from the groupconsisting of poly(vinylidenefluoride)-co-poly(hexafluoropropylene),poly(acrylonitrile), poly-(ethyleneoxide) and poly(alkylacrylate). 8.The cell according to claim 7, wherein the polymer gel electrolytecontains one or more polymers in an amount of 5-20% by weight of asolvent mixture of propylene carbonate and ethylene carbonate.
 9. Amethod for fabricating a dye-sensitized solar cell, comprising the stepsof: adding titanium isopropoxide into a polymer solution, followed byadding acetic acid as a catalyst thereinto, and then stirring theresulting mixture at room temperature, to obtain a solution forelectrospinning; electrospinning the solution to form a film made ofultra-fine titanium dioxide fibers onto an ITO or FTO-coated transparentconductive glass substrate; pre-treating the substrate having the filmof the ultra-fine titanium dioxide fibers formed thereon with acetone ordimethyl formamide; thermally treating the pre-treated substrate to forma layer of the ultra-fine fibrous titanium dioxide onto the substrate;impregnating the thermally treated substrate in a solution of dyemolecules in ethyl alcohol to obtain a semiconductor electrode on whichthe dye molecules are adsorbed into the electrospun ultra-fine titaniumdioxide fibers; coating a platinum layer onto an ITO or FTO-coatedtransparent conductive glass substrate, to obtain a counter electrode;performing a heating/pressing process on the semiconductor electrode andthe counter electrode with a spacer having a thickness of 20 μmtherebetween, so as to attach the semiconductor electrode and thecounter electrode to each other; and injecting electrolyte into emptyspace between the semiconductor electrode and the counter electrode. 10.The method according to claim 9, wherein the polymer solution is asolution in which a polymer selected from the group consisting ofpolyvinylacetate, polyvinylpyrrolidone, polyvinylalcohol andpolyethyleneoxide is dissolved in dimethyl formamide, acetone,tetrahydrofuran, toluene or a mixture thereof in an amount of 5-20 wt %.11. The method according to claim 9, wherein the electrospinning isperformed so as to make the ultra-fine titanium dioxide fibers have athickness of 50-1000 nm.
 12. The method according to claim 9, whereinthe layer of the ultra-fine fibrous titanium dioxide is formed at athickness of 5-20 μm after the thermal treatment.
 13. The methodaccording to claim 9, wherein the pre-treatment is performed by a methodi) the substrate is treated with vapor of acetone or dimethyl formamidefor 1-3 hours in a closed container, ii) the substrate is immersed inacetone or dimethyl formamide solvent for 1 hour or iii) that methods i)and ii) are combined.
 14. The method according to claim 9, wherein theelectrolyte is a liquid electrolyte or polymer gel electrolyte.